The present invention relates to a rotary-wing aircraft, and more particularly to a drive arrangement for a high speed compound or coaxial contra-rotating rotor aircraft in which a translational propulsion system provides translational thrust while the main rotor system is operated at a reduced airspeed in a reverse airflow condition during high speed flight.
The forward airspeed of a conventional rotary wing aircraft is limited by a number of factors. Among these is the tendency of the retreating blade to stall at high forward airspeeds. As the forward airspeed increases, the airflow velocity across the retreating blade slows such that the blade may approach a stall condition. In contrast, the airflow velocity across the advancing blade increases with increasing forward speed. Dissymmetry of lift is thereby generated by forward movement of the helicopter.
This dissymmetry may create an unstable condition if lift is not equalized across the advancing and retreating sectors of the rotor disc. Typically, blade flapping and feathering are utilized to generally equalize the lift.
However, as the forward airspeed is increased beyond a given point for a given rotor rpm, the flapping and feathering action eventually becomes inadequate to maintain substantial equality of lift over the rotor disc. At this point, reverse airflow across the retreating blade creates negative lift and, depending on the forward speed, creates a stalling or negative lift condition that travels outwardly across the blade as airspeed increases. Conventional rotors must be operated at airspeeds lower than those which cause reverse airflow across a substantial part of the retreating blade and at an rpm lower than that which would cause compressibility Mach number problems at the tip of the advancing blade. This has effectively limited forward airspeeds of conventional helicopters to approximately 180 knots.
A rotary wing aircraft with a coaxial contra-rotating rigid rotor system is capable of higher speeds compared to conventional single rotor helicopters due in part to the balance of lift between the advancing sides of the main rotor blades on the upper and lower rotor systems. In addition, the retreating side of the rotor discs are also generally free from classic retreating blade stall that conventional single or tandem rotor helicopters may suffer from.
To still further increase airspeed, a compound or coaxial contra-rotating rigid rotor aircraft operates a system in autorotation with supplemental translational thrust being provided by turbojet engines. In high speed flight, the main rotor system is unloaded from the main rotor drive engines (or turboshafts), and means for controlling rotor RPM is limited to adjusting collective pitch. For any helicopter in autorotation increasing collective pitch slows the rotational speed and decreasing collective pitch increases rotational speed. For a rotary wing aircraft in a high speed flight profile, however, rotor RPM is preferably decreased to prevent the rotor blade tips on the advancing sides of the rotor discs from entering a supersonic region as the aircraft airspeed increases. The necessary RPM reduction from hover to high speed is typically on the order of 30%. Generally speaking, autorotation is a rotary wing flight condition where the force to turn the blades comes from airflow to the underside of the rotors. The source of this airflow generally is from either the downward motion of an aircraft, such as would happen after engine failure, or forward motion of an aircraft, such as level flight in an autogiro.
As airspeed increases, collective pitch is increased to prevent the rotor RPM from increasing to an undesirable level. This requires the advancing side angle of attack (AOA) to increase as speed increases, which in turn generates more lift, more induced drag and a larger bending moment on the shaft of the main rotor. The increased lift is generally balanced by the retreating side of the rotor disc. Because an inboard portion of the retreating side is in reverse flow, and because of the collective pitch, the AOA goes negative. This generates negative lift. The moment generated from the increased lift on the advancing side adds to the moment generated from the negative lift on the retreating side. This moment is generally canceled by an equal and opposite moment by the other rotor in a coaxial, contra-rotating rotor system. However, any variation in the phase or magnitude of the upper and lower rotor system generates vibration that is propagated to the rest of the airframe.
Accordingly, it is desirable to provide a rotor drive and control system for a high speed rotary-wing aircraft which minimizes a major source of vibration and commensurate performance degradation.